In order to study the regulation of galactosyltransferase gene in normal and neoplastic mammary glands we set out to clone the cDNA corresponding to this protein. To date we have generated a library of cDNA clones from a rat lactating mammary gland. This library was hybridized to cloned cDNA probes for abundant class of rat mammary gland mRNA. cDNA clones which did not hybridize to these probes (negative selection) were further selected by hybridization to cDNA probes synthesized either from 14 day lactating rat mammary gland RNA and or from 2 day pregnant rat mammary gland RNA. A set of clones were selected by this procedure. These clones were then classified by the protein products synthesized in vitro by their complementary mRNAs. Two sets of cDNA clones were identified which are under hormonal control and have different levels of mRNAs in mammary tumors.